"Oh, I just made a silly mistake..."
I was so tired of always hearing that! In order to make sure that students are learning when they are retesting, I now get them to make their corrections on a "fix it sheet". This two page document gets printed front to back and stapled to their test. On the fix it sheet, students record the number of the problem they missed, what the correct answer should be, and either why their original answer was wrong or why the new answer is correct. That way they can't just say it was a silly mistake and guess another answer.